Fireflies after dark: Flashy nuptial gifts

Ezine

Published: Jul 1, 2012

Author: David Bradley

Channels: Infrared Spectroscopy

A protein fit for a king

Credit: Terry Priest

Once the lights go out, female fireflies apparently prefer a little more substance and a little less flash. Infrared imaging and other techniques have been used to monitor firefly behaviour and to show that the females of the species tend to choose mates that they perceive as able to deliver a large "nuptial gift" a high protein sperm package that helps females produce more eggs.

Earlier work on Photinus fireflies has demonstrated that the female of the species is very choosy during the on-the-wing stage of the courtship. Females only flash a response at select males that light up with especially attractive courtship flashes. However, it now seems that after a lengthy back-and-forth exchange, the flashing stops, the lights go out, and the firefly pairs mate. There is more than a delivery of sperm to the female that occurs though. The males give the females spermatophores, which contains protein nutrients that help the female produce more eggs. Despite this gift, the females may find an alternative mate the next night and ultimately mate several times before allowing particular sperm to fertilise their eggs. The question for evolutionary scientists is which male gets to pass his genes on to the next generation of firefly offspring, the generous donor of nuptial gifts or one of the subsequent mates?

"Lots of people don't realize that sexual selection is happening not only before mating, but also during and even after mating," explains Sara Lewis of Tufts University's School of Arts and Sciences. She and her colleagues have published details of their study online in Proceedings of the Royal Society - Biological Sciences in which they show that the more generous males who provide a nuptial gift are more likely to succeed in becoming the fathers of the next firefly generation. "Focusing on what happens after contact, we wanted to examine how much a male's success - in both mating and fathering offspring - depended on his flashes or on his nuptial gift offering."

Flashy and rich?

Lewis and team member Adam South used infrared video and paternity testing based on firefly DNA to determine which males were most successful after dark. Intriguingly, the team used programmed LED lights to simulate male firefly flashes. The team exposed one group of females to a flash pattern that earlier research had shown was highly attractive to females; second group saw only "unattractive" flash patterns. They also divided the males into two groups: those who had a large spermatophore to present, the virgins, and the experienced old-timers who had a smaller package.

A firefly sex tape?

The team ran several minutes of LED courtship flashing and then put males and females together in pairs and recorded video of the close-up courtship behaviour under infrared illumination. This behaviour had not been observed before because it always takes place in the dark. An analysis of several hours of firefly video revealed that once a female was in close proximity to a male, she was much more likely to mate with males that had larger nuptial gifts to offer, as determined by the researchers' later examination. The females didn't seem to care what kind of male flashes they had seen. "We were surprised to discover that attractive flashes only seem to benefit males during the early stages of firefly courtship," explains South. "Initially, flashes are important. Female fireflies preferentially respond to males based on temporal flash characteristics." Apparently, the males make physical contact through flashing, but the females then switch to an alternative cue - one that's related to male nuptial gift size. The most odd aspect of this finding is that as far as we know the females do not have any way to known what size of gift is likely to be offered as it hidden from view and transferred internally, it's almost like sexual reproduction by blind auction. Moreover, when females mated sequentially with two different males, paternity testing of their offspring revealed yet another benefit for big gift-givers. Males that gave larger nuptial gifts sired more offspring than rival males. This might be a simple fact of statistics, South and Lewis say, a bigger nuptial gift would most likely contain more sperm cells.